Scientists have long wondered what killed a group of Eocene animals, whose well-preserved remains have been recovered over the past few decades from the Messel Lake deposits in Germany. Now researchers at the University of Bonn Institute for Paleontology have found evidence that many of the animals were poisoned by toxin-releasing bacteria.

The 47-million-year-old fossils from the World Heritage Site of Messel, near Darmstadt, show astonishing diversity, from dog-sized horses to birds, bats, beetles, and plants. Scientists speculated that the animals died from noxious volcanic gases because the lake is thought to have been in an ancient volcanic basin. However, the presence of birds and bats in the mix led some to question this theory, as these creatures should have been able to fly away from the deadly vapors.

Now paleontologist Wighart von Koenigswald and colleagues, reporting in the latest issue of Paläontologische Zeitschrift, claim to have solved the riddle. The team noticed that an unusual number of specimens of the primitive horse Propalaeotherium (five out of 50) were pregnant mares. They also found five intimately associated pairs of Allaeochelys turtles, perhaps mating when they died. This suggested a link between early summer reproduction and death in the lake.

By comparing the Messel sediments with those of other ancient and modern lakes, the researchers determined that cyanobacteria were abundant in the ancient lake. These bacteria release toxins when water nitrogen levels are high--a condition more prevalent in the summer--and "animals drinking such poisoned water die almost instantly," observes Koenigswald's team.

This is a "provocative new model for the death of the Messel mammals," says Yale University paleontologist Derek Briggs. "It should stimulate new research to detect any correlation between death assemblages and cyanobacterial blooms."